Overall survival in children with acute myeloid leukemia (AML) has improved to 60-70%. However, after disease recurrence, the likelihood of long-term survival is poor (20-30%). Several subtypes of childhood AML are at high risk of relapse including a group with internal tandem duplication (ITD) mutations in the receptor tyrosine kinase FLT3. Children with newly diagnosed FLT3-ITD-postive AML have an overall survival of 40% when treated with induction therapy followed by 3-4 courses of chemotherapy, whereas worse survival rates have been reported in adults with FLT3-ITD mutated AML. FLT3-ITD mutations occur in about 15% of pediatric AML and more than 30% of adult AML. Further significant improvements in long-term survival of FLT3-ITD-positive AML are not expected with conventional chemotherapy alone and new therapeutic strategies are needed. In this proposal, we outline three related projects to circumvent and treat drug-resistant FLT3-ITD- positive AML including: (Aim 1) to determine the effects of sorafenib in combination with crenolanib on drug efficacy, toxicity, PK, PD and resistance profiles in relapsed/refractory pediatric FLT3-ITD+ AML, with our hypothesis that optimal FLT3 inhibition is required to suppress the emergence of TKI-resistant secondary TKD mutations; (Aim 2) to determine the role of the Tec kinase BMX in sorafenib resistance, with our hypothesis that during FLT3 inhibition, BMX upregulation and activation provides a compensatory signaling pathway that confers resistance to sorafenib; and (Aim 3) to identify effective sorafenib combinations in FLT3-ITD+ AML. In the latter Aim, the translational potential of ibrutinib, a lead BMX inhibitor, when given in combination with sorafenib, will be evaluated, with our hypothesis that ibrutinib will suppress BMX activity during sorafenib treatment and in combination will be an effective treatment strategy for FLT3-ITD+ AML. Our strategy represents a continuous interplay between laboratory in vitro and in vivo experimental approaches and clinical observations, and we hypothesize that this approach will lead to the identification of important, previously unrecognized mechanisms of TKI resistance as well as to the future discovery of novel treatment strategies for childhood AML with improved outcome.